Made available in DSpace on 2014-06-11T19:34:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-12Bitstream added on 2014-06-13T19:44:43Z : No. of bitstreams: 1 silva_ap_dr_guara.pdf: 3551125 bytes, checksum: 3f75e8c0e99826910556f8cd772c1403 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho analisa a transformação da austenita retida induzida por deformação, em um aço AISI 4340 com estruturas multifásicas. O aço foi submetido a duas rotas de tratamentos isotérmicos selecionadas e foi analisado quanto à fração volumétrica das fases e características mecânicas através da valiação dos limites de escoamento. Os corpos de prova foram submetidos a três diferentes níveis de tensões de tração, acima do limite de escoamento, para que se produzissem deformações plásticas determinadas. A influência destas deformações, na microestrutura e nos novos limites de escoamento, foram avaliadas. Mostrando variações significativas nas frações volumétricas das fases, com diminuição das quantidades de austenita, indicando uma transformação, por efeito TRIP (Transformation Induced Plasticity), em martensita o que provocou um aumento nos valores dos limites de escoamento e envelhecimento por deformação. / The Transformation induced plasticity of austenite on the multiphase microstructure in a AISI 4340 steel was studied. Two sequences of isothermal treatments were performed. The volume fractions of the phases were analyzed and evaluation of mechanical characteristics through the yield point measurements. The samples were submitted at three different levels of traction above of the yield strength, so that definitive plastic deformations were produced. The influence of these deformations in the microstructure and in the new yield strength was evaluated. Significant variations were showed in the phase volume fraction, with reduction of the amounts of austenite indicating a martensitic transformation, by effect TRIP (Transformation Induced Plasticity) which results increases in the values of the yield strength and deformation ageing.

The Quenching and Partitioning (Q&P) heat treatment has been proposed in the early 2000s to produce cold-rolled sheets combining high-strength and formability for the automotive market. Q&P consists, first, of an interrupted quench between the martensite-start temperature (Ms) and the martensite-finish temperature (Mf) from intercritical annealing or full austenitization in order to form controlled fractions of martensite. This is followed by a partitioning step in order to stabilize the untransformed austenite through carbon enrichment. In order to maximize the carbon transfer from martensite to austenite, the use of specific alloying elements and the design of appropriate Q&P parameters are required to eliminate or minimize competing phenomena such as carbide formation and austenite decomposition. The final quenched and partitioned microstructure, using full austenitization, ideally consists of carbon-depleted lath martensite and significant fractions of retained austenite providing an improved combination of strength and ductility. Hence, the transformation of retained austenite upon straining at room temperature (TRIP effect) provides supplementary work-hardening and eventually improves the ductility. In the present work, Quenching and Partitioning (Q & P) heat treatments were carried out in a quench dilatometeron a 0.2 wt% carbon steel. The microstructure evolution of the Q & P steels was characterized usingdilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate thefraction of martensite formed at a given temperature below the martensite start temperature Ms. Q & P wasshown to be an effective way to stabilize retained austenite at room temperature. However, the measuredaustenite fractions after Q & P treatments showed significant differences when compared to the calculated valuesconsidering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitiveto the quench temperature and were never larger than the ideal predicted maximum fraction. Competitivereactions such as austenite decomposition into bainite and carbide precipitation were found to occur in thepresent work.Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperaturesand partitioning times. The direct contributions between Q & P microstructural constituents -such as retainedaustenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This wascritically discussed and compared to quenching and austempering (QAT) which is a more conventional processingroute of stabilizing retained austenite at room temperature. Finally, Q & P steels were shown to exhibit aninteresting balance between strength and ductility. The achievement of this interesting combination of mechanicalproperties was reached for much shorter processing times compared to QAT steels. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Orientador: Tomaz Manabu Hashimoto / Banca: Marcelo dos Santos Pereira / Banca: Antonio Jorge Abdalla / Banca: Pedro Paulo de Campos / Banca: José Rubens de Camargo / Resumo: Este trabalho analisa a transformação da austenita retida induzida por deformação, em um aço AISI 4340 com estruturas multifásicas. O aço foi submetido a duas rotas de tratamentos isotérmicos selecionadas e foi analisado quanto à fração volumétrica das fases e características mecânicas através da valiação dos limites de escoamento. Os corpos de prova foram submetidos a três diferentes níveis de tensões de tração, acima do limite de escoamento, para que se produzissem deformações plásticas determinadas. A influência destas deformações, na microestrutura e nos novos limites de escoamento, foram avaliadas. Mostrando variações significativas nas frações volumétricas das fases, com diminuição das quantidades de austenita, indicando uma transformação, por efeito TRIP (Transformation Induced Plasticity), em martensita o que provocou um aumento nos valores dos limites de escoamento e envelhecimento por deformação. / Abstract: The Transformation induced plasticity of austenite on the multiphase microstructure in a AISI 4340 steel was studied. Two sequences of isothermal treatments were performed. The volume fractions of the phases were analyzed and evaluation of mechanical characteristics through the yield point measurements. The samples were submitted at three different levels of traction above of the yield strength, so that definitive plastic deformations were produced. The influence of these deformations in the microstructure and in the new yield strength was evaluated. Significant variations were showed in the phase volume fraction, with reduction of the amounts of austenite indicating a martensitic transformation, by effect TRIP (Transformation Induced Plasticity) which results increases in the values of the yield strength and deformation ageing. / Doutor

For ecological reasons, the current main challenge of the automotive industry is to reduce the fuel consumption of vehicles and then emissions of greenhouse gas. In this context, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as TRIP steels. A combination of high strength and large elongation is obtained thanks to the TRansformation Induced Plasticity (TRIP) effect. However, improvement of the mechanical properties is still possible, especially by the refinement of the matrix. In this work, two main ways were followed in order to reach improved properties. The classical way consisting of the annealing of cold-rolled samples and an innovative way consisting of obtaining the desired microstructure by direct hot rolling of the samples. In the classical way, this refinement can be obtained by acting on the chemical composition (with such alloying elements like Cu and Nb). It was observed that complete recrystallisation of the ferrite matrix is quite impossible in presence of Cu precipitates. In addition, if the ferrite recrystallisation is not completed before reaching the eutectoid temperature, the recrystallisation will be slowed down by a large way. An innovative heat treatment consisting in keeping the copper in solid solution in the high-Cu steel was developed. Therefore, ferrite recrystallises quite easily and very fine ferrite grains (~1µm) were obtained. In the innovative way, the effects of hot-rolling conditions on TRIP-assisted multiphase steels are of major importance for industrial practice and could open new dimensions for the TRIP steels (i.e. thanks to precipitation mechanisms leading to additive strengthening). Impressive mechanical properties (true stress at maximum load of 1500 MPa and true strain at uniform elongation of 0.22) were obtained with a relatively easy thermomechanical process, the role played by Nb being essential.

For ecological reasons, the current main challenge of the automotive industry is to reduce the fuel consumption of vehicles and then emissions of greenhouse gas. In this context, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as TRIP steels. A combination of high strength and large elongation is obtained thanks to the TRansformation Induced Plasticity (TRIP) effect. However, improvement of the mechanical properties is still possible, especially by the refinement of the matrix. In this work, two main ways were followed in order to reach improved properties. The classical way consisting of the annealing of cold-rolled samples and an innovative way consisting of obtaining the desired microstructure by direct hot rolling of the samples. In the classical way, this refinement can be obtained by acting on the chemical composition (with such alloying elements like Cu and Nb). It was observed that complete recrystallisation of the ferrite matrix is quite impossible in presence of Cu precipitates. In addition, if the ferrite recrystallisation is not completed before reaching the eutectoid temperature, the recrystallisation will be slowed down by a large way. An innovative heat treatment consisting in keeping the copper in solid solution in the high-Cu steel was developed. Therefore, ferrite recrystallises quite easily and very fine ferrite grains (~1µm) were obtained. In the innovative way, the effects of hot-rolling conditions on TRIP-assisted multiphase steels are of major importance for industrial practice and could open new dimensions for the TRIP steels (i.e. thanks to precipitation mechanisms leading to additive strengthening). Impressive mechanical properties (true stress at maximum load of 1500 MPa and true strain at uniform elongation of 0.22) were obtained with a relatively easy thermomechanical process, the role played by Nb being essential.

Pour répondre à la demande croissante d’allègement des véhicules automobiles, les aciéristes développent une nouvelle gamme d’aciers à Très Haute Résistance (THR), dite de troisième génération. Cette thèse, inscrite dans le projet ANR MATETPRO « MeMnAl Steels », s’intéresse plus particulièrement à deux nouvelles familles d’aciers THR Fe-C-Mn-Al, produites par ArcelorMittal et potentielles candidates pour la caisse en blanc des futurs véhicules. Elle vise à mieux cerner les paramètres microstructuraux permettant de contrôler et optimiser le comportement à rupture de ces aciers.Pour représenter les deux familles d’aciers, deux matériaux modèles ont été élaborés par laminage puis recuit intercritique, conduisant à une microstructure duplex : austénite retenue (γr, pouvant se transformer en martensite par effet TRIP) et ferrite. La microstructure du premier acier, dite UFG, est ultrafine (grains de taille inférieure au micromètre) tandis que celle du second est bimodale, mêlant gros grains de ferrite δ et régions à grains fins de ferrite α et d’austénite retenue γr.Les propriétés mécaniques de la microstructure UFG dépendent fortement de la température de recuit, en raison des variations de stabilité de l’austénite retenue. A l’inverse, la microstructure bimodale est très robuste vis-à-vis de la température de recuit mais très sensible à la température d’essai. L’endommagement en traction et en résilience est très peu développé pour ces deux familles. Il est localisé aux interfaces ferrite-martensite (formée pendant l’essai). Le lien entre les modes de rupture et la microstructure bimodale, étudié à l’aide d’essais Charpy, a montré l’existence de deux transitions distinctes de mode de rupture : une transition entre rupture ductile à grandes cupules et clivage pour les gros grains de ferrite δ et une transition entre rupture interfaciale et rupture ductile à fines cupules pour les zones à grains fins {α + γr}. La rupture de la microstructure UFG est ductile à température ambiante et interfaciale à plus basse température. Cette microstructure UFG peut être vue comme un matériau modèle représentant les régions à grains fins {α + γr} de la microstructure bimodale.Pour les deux familles d’aciers, le comportement élastoplastique comme le comportement à rupture semblent dominés par la stabilité de l’austénite retenue. / To fulfil the increasing demand on lightweighting automotive vehicles, steelmakers are developing a third generation of Advanced High Strength Steels (AHSS). This work, part of the ANR project MATETPRO “MeMnAl Steels”, addressed two new families of third generation AHSS produced by ArcelorMittal which may be used for the body in white of upcoming cars. It aimed at improving our current understanding of the microstructural features allowing controlling and optimizing the fracture behavior of this steel family.Two model materials were manufactured by hot and cold rolling followed by intercritical annealing. The resulting, so-called duplex microstructure is a mixture of ferrite and retained austenite (γr, which can transform into martensite by TRIP effect). The microstructure of the first steel was made of ultra-fine grains (UFG) of ferrite and retained austenite (grain size below one micrometer), while the second steel possessed a bimodal microstructure made of coarse δ-ferrite grains and fine-grained regions of α-ferrite and retained austenite γr.The mechanical properties of the UFG microstructure were strongly sensitive to the annealing temperature, due to variations in the stability of retained austenite. On the contrary, the bimodal microstructure was very robust regarding the annealing temperature but very sensitive to the test temperature. For these two families, damage development is scarce and mainly located at ferrite-martensite interfaces. Charpy impact tests on steels with the bimodal microstructure showed that each microstructural region presents its own fracture mechanisms and a specific ductile-to-brittle transition. A transition from brittle cleavage to large-dimpled, ductile fracture was observed for coarse δ-ferrite grains, while fined-grained regions presented a transition from interfacial fracture to fine-dimpled, ductile fracture. Fracture of the UFG microstructure was ductile at room temperature and interfacial at lower temperatures. This UFG microstructure can be interpreted as a model material embodying the behavior of the fine-grained {α + γr} regions in the bimodal microstructure.For both two steels, the constitutive and fracture behavior seem to be dominated by the stability of retained austenite.

A aplicação do novo conceito de tratamento térmico, chamado de têmpera e partição (Q&P), desenvolvido para a obtenção de aços da terceira geração da classe AHSS (Advanced high strengh steel ou aços avançados de alta resistência), mostra-se uma alternativa para o processamento de ferros fundidos nodulares com alta resistência mecânica. No processo Q&P, o carbono presente na martensita supersaturada, formada na etapa de têmpera, é utilizado para estabilizar a austenita não transformada durante a etapa de partição, mantendo-a estável na temperatura ambiente. Essa rota de tratamento térmico consiste em realizar uma têmpera no material (após uma etapa prévia de austenitização) numa faixa de temperatura entre o Ms e Mf (temperatura de início e fim da transformação martensítica, respectivamente), seguido de um reaquecimento e manutenção a uma temperatura acima do Ms (etapa isotérmica de partição) com o objetivo de que o carbono migre da martensita supersaturada para a austenita remanescente promovendo sua estabilização. Essa partição do carbono só é possível caso a precipitação da cementita seja suprimida, e isso é conseguido com a presença de elementos de liga, como o Si e/ou Al. Neste trabalho foi feito um estudo sobre as características microestruturais e mecânicas de um ferro fundido nodular (3,47%C; 2,47%Si; 0,2%Mn) submetido ao tratamento térmico de têmpera e partição, o qual foi denominado neste trabalho como, Q&PDI (Quenched and Partitioned Ductile Iron ou Ferro Fundido Nodular Temperado e Particionado). Para isso foi realizada uma austenitização plena nas amostras, a 880°C, por duas horas, seguida de uma têmpera em óleo pré-aquecido nas temperaturas de 140 e 170°C. A etapa de partição foi feita nas temperaturas de 300, 375 e 450°C, com intervalos de tempo variando de 5 a 120 minutos. A caracterização microestrutural foi realizada através de microscopia ótica, microscopia eletrônica de varredura e EBSD. A técnica de difração de raios-X foi empregada para quantificar a fração volumétrica e o teor de carbono na austenita retida. Ensaios de dilatometria, difração de raios-X \"in situ\" e nanoindentação foram empregados para auxiliar na análise das transformações de fases que ocorreram na etapa de partição, como a transformação bainítica e a precipitação de carbonetos de transição nas placas de martensita. A caracterização mecânica foi feita através de ensaios de tração, impacto, dureza, nanoindentação, tenacidade à fratura e resistência à fadiga. Ensaios de compressão auxiliaram na análise da transformação martensítica induzida por deformação. Os resultados obtidos mostraram que é possível obter ferros fundidos nodulares com alta resistência mecânica (limite de resistência >1450 MPa), com consideráveis ductilidade (de até 9%) e energia absorvida sob impacto (de até 81 J), bem como tenacidades à fratura de 55 MPa.m1/2 e limites de fadiga de 550 MPa. Este comportamento é proporcionado por uma microestrutura singular, constituída por uma dispersão homogênea de placas de martensita numa matriz de ausferrita bastante refinada, com consideráveis frações volumétricas de austenita retida (max. 23%). / Quenching and partitioning (Q & P), a new heat treatment concept developed to obtain third generation AHSS (Advanced High Strength Steel), is an alternative for processing of nodular cast irons in order to obtain high mechanical strength. In the Q & P process, the carbon present in the supersaturated martensite formed in the quenching step diffuses towards the untransformed austenite during the partition step, keeping it stable at room temperature. This heat treatment route consists of quenching the material (after a previous step of austenitization) in a temperature range between Ms and Mf (beginning and end temperature of the martensitic transformation, respectively), followed by reheating and maintenance at a temperature above the Ms (isothermal stage of partition) allowing the carbon to migrate from the supersaturated martensite to the remaining austenite, promoting its stabilization. This partition of carbon is only possible if precipitation of cementite is suppressed; this is achieved adding alloying elements such as Si and/or Al. In this work a study was made on the microstructural and mechanical characteristics of a ductile iron (3.47%C; 2.47%Si; 0.2%Mn), submitted to a Q&P heat treatment, in this work named Q & PDI (Quenched and Partitioned Ductile Iron). A full sample austenitization was carried out at 880 ° C for two hours, followed by a pre-heated oil quanching at temperatures of 140 and 170 ° C. The partitioning step was at temperatures of 300, 375 and 450Â ° C, with time intervals ranging from 5 to 120 minutes. Microstructural characterization was performed through optical microscopy, scanning electron microscopy and EBSD. The X-ray diffraction technique was used to quantify the volumetric fraction and the carbon content in the retained austenite. Dilatometry, X-ray diffraction \"in situ\" and nanoindentation were also used to aid in the analysis of the phase transformations that occurred in the partitioning stage, such as the bainitic transformation and the precipitation of transition carbides in the martensite plates. Mechanical characterization was performed through tensile, impact, hardness, nanoindentation, fracture toughness and fatigue strength tests. Compression tests aided in the analysis of the deformation induced martensitic transformation. The results showed that it is possible to obtain nodular cast irons with high mechanical strength (resistance limit> 1450 MPa), with considerable ductility (up to 9%) and energy absorbed under impact (up to 81 J), as well as fracture toughness of 55 MPa.m -1 / 2 and fatigue limits of 550 Mpa. This behavior is provided by a unique microstructure, consisting of a homogeneous dispersion of martensite plates in a very refined ausferrite matrix, with considerable volumetric fractions of retained austenite (max. 23%).

A aplicação do novo conceito de tratamento térmico, chamado de têmpera e partição (Q&P), desenvolvido para a obtenção de aços da terceira geração da classe AHSS (Advanced high strengh steel ou aços avançados de alta resistência), mostra-se uma alternativa para o processamento de ferros fundidos nodulares com alta resistência mecânica. No processo Q&P, o carbono presente na martensita supersaturada, formada na etapa de têmpera, é utilizado para estabilizar a austenita não transformada durante a etapa de partição, mantendo-a estável na temperatura ambiente. Essa rota de tratamento térmico consiste em realizar uma têmpera no material (após uma etapa prévia de austenitização) numa faixa de temperatura entre o Ms e Mf (temperatura de início e fim da transformação martensítica, respectivamente), seguido de um reaquecimento e manutenção a uma temperatura acima do Ms (etapa isotérmica de partição) com o objetivo de que o carbono migre da martensita supersaturada para a austenita remanescente promovendo sua estabilização. Essa partição do carbono só é possível caso a precipitação da cementita seja suprimida, e isso é conseguido com a presença de elementos de liga, como o Si e/ou Al. Neste trabalho foi feito um estudo sobre as características microestruturais e mecânicas de um ferro fundido nodular (3,47%C; 2,47%Si; 0,2%Mn) submetido ao tratamento térmico de têmpera e partição, o qual foi denominado neste trabalho como, Q&PDI (Quenched and Partitioned Ductile Iron ou Ferro Fundido Nodular Temperado e Particionado). Para isso foi realizada uma austenitização plena nas amostras, a 880°C, por duas horas, seguida de uma têmpera em óleo pré-aquecido nas temperaturas de 140 e 170°C. A etapa de partição foi feita nas temperaturas de 300, 375 e 450°C, com intervalos de tempo variando de 5 a 120 minutos. A caracterização microestrutural foi realizada através de microscopia ótica, microscopia eletrônica de varredura e EBSD. A técnica de difração de raios-X foi empregada para quantificar a fração volumétrica e o teor de carbono na austenita retida. Ensaios de dilatometria, difração de raios-X \"in situ\" e nanoindentação foram empregados para auxiliar na análise das transformações de fases que ocorreram na etapa de partição, como a transformação bainítica e a precipitação de carbonetos de transição nas placas de martensita. A caracterização mecânica foi feita através de ensaios de tração, impacto, dureza, nanoindentação, tenacidade à fratura e resistência à fadiga. Ensaios de compressão auxiliaram na análise da transformação martensítica induzida por deformação. Os resultados obtidos mostraram que é possível obter ferros fundidos nodulares com alta resistência mecânica (limite de resistência >1450 MPa), com consideráveis ductilidade (de até 9%) e energia absorvida sob impacto (de até 81 J), bem como tenacidades à fratura de 55 MPa.m1/2 e limites de fadiga de 550 MPa. Este comportamento é proporcionado por uma microestrutura singular, constituída por uma dispersão homogênea de placas de martensita numa matriz de ausferrita bastante refinada, com consideráveis frações volumétricas de austenita retida (max. 23%). / Quenching and partitioning (Q & P), a new heat treatment concept developed to obtain third generation AHSS (Advanced High Strength Steel), is an alternative for processing of nodular cast irons in order to obtain high mechanical strength. In the Q & P process, the carbon present in the supersaturated martensite formed in the quenching step diffuses towards the untransformed austenite during the partition step, keeping it stable at room temperature. This heat treatment route consists of quenching the material (after a previous step of austenitization) in a temperature range between Ms and Mf (beginning and end temperature of the martensitic transformation, respectively), followed by reheating and maintenance at a temperature above the Ms (isothermal stage of partition) allowing the carbon to migrate from the supersaturated martensite to the remaining austenite, promoting its stabilization. This partition of carbon is only possible if precipitation of cementite is suppressed; this is achieved adding alloying elements such as Si and/or Al. In this work a study was made on the microstructural and mechanical characteristics of a ductile iron (3.47%C; 2.47%Si; 0.2%Mn), submitted to a Q&P heat treatment, in this work named Q & PDI (Quenched and Partitioned Ductile Iron). A full sample austenitization was carried out at 880 ° C for two hours, followed by a pre-heated oil quanching at temperatures of 140 and 170 ° C. The partitioning step was at temperatures of 300, 375 and 450Â ° C, with time intervals ranging from 5 to 120 minutes. Microstructural characterization was performed through optical microscopy, scanning electron microscopy and EBSD. The X-ray diffraction technique was used to quantify the volumetric fraction and the carbon content in the retained austenite. Dilatometry, X-ray diffraction \"in situ\" and nanoindentation were also used to aid in the analysis of the phase transformations that occurred in the partitioning stage, such as the bainitic transformation and the precipitation of transition carbides in the martensite plates. Mechanical characterization was performed through tensile, impact, hardness, nanoindentation, fracture toughness and fatigue strength tests. Compression tests aided in the analysis of the deformation induced martensitic transformation. The results showed that it is possible to obtain nodular cast irons with high mechanical strength (resistance limit> 1450 MPa), with considerable ductility (up to 9%) and energy absorbed under impact (up to 81 J), as well as fracture toughness of 55 MPa.m -1 / 2 and fatigue limits of 550 Mpa. This behavior is provided by a unique microstructure, consisting of a homogeneous dispersion of martensite plates in a very refined ausferrite matrix, with considerable volumetric fractions of retained austenite (max. 23%).

Les aciers inoxydables duplex présentent une combinaison intéressante entre des propriétés mécaniques élevées, une faible conductivité thermique et un coût relativement faible. Ils sont couramment employés dans le domaine du bâtiment comme rond à béton, application qui requière notamment une résistance élevée (Rm > 950 MPa) et une ductilité importante (A% > 15). Cette thèse a pour objectif d’améliorer le compromis résistance / allongement, en développant de nouvelles nuances duplex présentant une transformation martensitique induite par la plasticité (effet TRIP) aux caractéristiques contrôlées. L’optimisation de ce compromis a nécessité en particulier une compréhension détaillée des mécanismes de transformation et de déformation plastique associés à chaque phase : la ferrite (BCC), l’austénite (FCC) et la martensite (BCC).L’influence de la transformation martensitique sur le comportement mécanique est étudiée pour quatre alliages duplex de stabilité variable de la phase austénitique en fonction de leur composition chimique. L’influence d’une microstructure multiphasée sur la cinétique de transformation est déterminée grâce à l’élaboration de trois nuances modèles représentant respectivement une nuance duplex et es deux compositions représentatives de ses constituants austénite et ferrite. L’utilisation de plusieurs techniques de caractérisation à différentes échelles a permis de décrire à la fois les mécanismes de transformation de phase et leur cinétique en fonction de la déformation, donnant ainsi accès à leur influence sur le comportement mécanique. L’étude des champs cinématiques a mis en évidence l’impact de la phase martensitique sur la répartition des déformations dans la microstructure multi-phasée. Finalement l’utilisation d’un modèle mécanique prenant en compte explicitement la transformation martensitique a permis de reproduire le comportement mécanique d’un alliage duplex. / Duplex stainless steels offer an attractive combination of high mechanical properties, low thermalconductivity and a relatively low cost. They are increasingly used as structural materials such as inthe construction sector as concrete reinforcement bars, where both high strength (Rm > 900 MPa)and high elongation to failure (A% > 15 %) are required. This thesis aims at improving the strength/ elongation compromise by developing new duplex stainless steel compositions experiencing a wellcontrolledmartensitic transformation induced by plasticity (TRIP effect). The optimisation of thiscompromise has required a good understanding of the transformation mechanisms and of plasticdeformation associated with each phase : ferrite (BCC), austenite (FCC) and martensite (BCC).The influence of martensitic transformation on mechanical behavior has been studied in four duplexgrades of variable austenite stability as a function of their chemical composition. The influence ofmultiphase microstructure on martensitic transformation kinetics has been determined by makingthree alloys respectively representative of a duplex grade and its two constituents (austenite andferrite). Using multiple characterization techniques at different scales has allowed determiningboth the transformation mechanisms and its kinetics as a function of strain, giving thus accessto the influence of transformation on the mechanical behavior. The study of kinematic fields hashighlighted the impact of the martensitic phase on the distribution of deformations. Finally, theuse of a mechanical model taking explicitly into account the phase transformation has allowed theduplication of the mechanical behavior of a duplex stainless steel.